Many processes are known wherein fibers from a plurality of positions are deposited and intermingled on the surface of a moving collection surface to form a wide nonwoven sheet. For instance, U.S. Pat. No. 3,402,227 (Knee) discloses a plurality of single orifice jets positioned above a receiver and spaced in a line that makes an angle with the direction of receiver movement so that the fiber streams that issue from the jets deposit fibers on discrete areas of the receiver to form ribbons which combine with ribbons formed from other streams along the line.
Several methods are known for directing the fibers from a plurality of positions to various locations across the width of the collection surface. For example, U.S. Pat. No. 3,169,899 (Steuber) discloses the use of curved oscillating baffles for spreading flashspun plexifilamentary strands while oscillating and directing them to a moving collection surface. Processes for flash-spinning plexifilamentary strands from a polymer spin solution are disclosed in U.S. Pat. Nos. 3,081,519 (Blades et al.) and 3,227,794 (Anderson et al.).
An efficient and improved method for depositing fibers onto the surface of a moving collection surface is disclosed in U.S. Pat. No. 3,497,918 (Pollock et al.). In a preferred embodiment of Pollock et al., plexifilamentary strands are flashspun and forwarded in a generally horizontal direction into contact with the surface of a rotating-lobed baffle. The baffle deflects the strand and accompanying expanded solvent gas downward into a generally vertical plane. Simultaneously, the baffle spreads the strands into a wide, thin web and causes the web to oscillate as it descends toward the collection surface. An electrostatic charge is imparted to the web during its descent to the collection surface. The web is then deposited as a wide swath on the surface of the collection surface. To make a wide sheet, numerous flash-spinning units of this type are employed. The units are positioned above the moving collection surface so that the deposited swaths form ribbons which partially overlap and combine to form a multi-layered sheet.
U.S. Pat. No. 4,537,733 (Farago) suggests that a multi-position apparatus of the type described in Pollock et al. be operated with the frequency of oscillation of the fiber streams varying by more than .+-.5%, but less than .+-.50% of the average oscillation frequency, in order to eliminate gage bands in the resulting nonwoven sheet. Gage bands do not necessarily produce a visual defect in the flat sheet itself, but are usually noticeable when a large roll is formed from the nonwoven sheet. The method of Farago and the apparatus of Pollock et al. have been modestly successful in reducing gage bands in the commercial production of wide nonwoven sheets prepared from flash-spun plexifilamentary strands. Such sheets are commercially available from E. I. du Pont de Nemours and Company under the trademark Tyvek.RTM. spunbonded polyolefin.
However, the utility of the nonwoven sheets could further be enhanced by improvements in sheet uniformity and appearance, particularly with regard to reducing the frequency and size of an undesired effect, referred to herein as "ropiness". Ropiness exhibits itself as agglomerated groups of fibers or fibrils that look like strings on the surface or within an otherwise uniform sheet. Ropes occur when a web becomes twisted or collapses on itself. Ropiness is especially apparent when viewed with a light source provided behind the sheet. Such nonuniformities often measure as much as 30 cm long and 1 cm wide and detract from the utility of the sheet, especially in end-uses that require printing on the sheet. The problems of ropiness and overall sheet quality become worse when the flow rate of the spin solution from individual spin positions is increased. This occurs since handling becomes inherently more difficult with larger and coarser webs. As flow rates are increased, the gas streams conveying the swaths interact more turbulently with each other and cause the uniformity of the resulting sheet to decrease.
Clearly, what is needed is an apparatus and method for economically maintaining or improving the uniformity of a nonwoven sheet as the flow rate of the spin solution from individual spin positions is increased. It is therefore an object of the present invention to provide an apparatus and method for making nonwoven sheets having less ropiness and improved uniformity at relatively high individual spin position flow rates. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.